The present invention relates to materials that are used to store gas molecules. More specifically, it relates to metal organic frameworks that have been treated to provide increased storage for carbon dioxide as compared to metal organic frameworks that have not been treated.
Carbon dioxide has been described as having a greater impact upon the environment than any other greenhouse gas due to the amount of carbon dioxide being added to the atmosphere by human activities. Carbon dioxide levels have increased by over 30% since the beginning of the Industrial Revolution. This increase has been calculated by scientists as being associated with a global warming trend and increased acidity of the oceans. About 75% of the increase in carbon dioxide levels is attributed to the burning of fossil fuels with the largest source of carbon dioxide emissions being from coal fired power plants, accounting for a total of about ⅓ of all CO2 emissions in the United States. One way to mitigate the amount of carbon dioxide that is being released is to capture it as it is produced at a coal-fired power plant either for short term use or long term capture and storage. A variety of materials have been tried for artificial carbon sequestration. Mitigation technologies are necessary for the short- and long-term capture and storage of carbon dioxide. Removal of carbon dioxide from the flue exhaust of power plants is commonly accomplished by chilling and pressurizing the exhaust or by passing the fumes through a fluidized bed of aqueous amine solution, both of which are costly and inefficient. Other methods based on chemisorption of carbon dioxide on oxide surfaces or adsorption within porous silicates, carbon, and membranes have been pursued as means for carbon dioxide uptake. Although each of the prior art technologies work to some extent, more cost effective technologies are necessary to cope with the overwhelming amount of carbon dioxide currently generated. The cost of current methods is about $150/ton of carbon. However, the known methods are too expensive to be commercially feasible for expanded use without significant government subsidies.
Metal organic-frameworks (MOFs) are a new class of nanoporous materials that have potential applications in separation processes, catalysis and gas storage. MOFs are synthesized using organic linker molecules and metal joints that self-assemble to form materials with well defined pores, high surface areas, and desired chemical functionalities. Because of these attractive properties, MOFs are promising candidates for CO2 capture. In the present invention, it is now shown that MOF Cu-BTC can be easily tuned to significantly enhance adsorption of CO2 by pre-adsorbing a small amount of water. This tuning for enhanced adsorption of CO2 applies to certain other guest molecules and other MOFs.
In many MOFs, the metal atoms are coordinatively saturated by the organic linkers that are connected to create the framework. However, in some MOFs, metal atoms are partially coordinated by solvent molecules from the synthesis procedure. It is common practice to activate MOFs at elevated temperature to remove the solvent and open up the void space for desired guest molecules. If the evacuation temperature is high enough, all solvent molecules can be removed, including those that are coordinatively bound to framework metal atoms. Removing these coordinated solvent molecules leaves coordinatively-unsaturated, open-metal sites that have been shown to promote high gas uptake, especially for H2 adsorption. Recently, for example, Bae et al. showed that in a carborane-based MOF removal of coordinated dimethylformamide (DMF) increased CO2 and CH4 adsorption and led to high selectivity for CO2 over methane. The open-metal sites in MOFs are reminiscent of the extra-framework cations in zeolites, in that they are expected to create large electric fields and to eagerly bind polar molecules. In zeolites, it is well known that the presence of water significantly decreases the adsorption of CO2 because water competitively adsorbs on the cations, blocking access for CO2. In this work, the opposite effect is found: water bound to open-metal sites substantially increases CO2 adsorption.
Cu-BTC (also known as HKUST-1) is a well-studied MOF first synthesized by Chui et al. The structure is composed of large central cavities (diameter 9.0 Å) surrounded by small pockets (diameter 5.0 Å), connected through triangular-shaped apertures of similar size. The Cu-BTC framework has paddlewheel type metal corners connected by benzene-1,3,5-tricarboxylate (BTC) linkers. Each metal corner has two copper atoms bonded to the oxygens of four BTC linkers. In the as-synthesized material, each copper atom is also coordinated to one water molecule. Metal-organic frameworks (MOFs) are a recent addition to the classes of porous materials and have the potential for providing just such a flexible platform for developing designer adsorbents. MOFs are synthesized by self-assembly of organic ligands and metal oxide clusters. The resulting crystalline materials possess regular porous structures with pore sizes and chemical functionalities that can be manipulated by modifying the metal group. Metal organic frameworks have been found to have the capacity to capture CO2 readily and at high selectivity over other gases such as nitrogen. In US 2007/0068389, there are described a number of MOFs that have the capacity to store CO2. This is a promising technology, and would be even more promising if it had even higher capacity for storage of CO2. Some MOFs possess open metal sites and can coordinate CO2 or other molecules more strongly than typical physisorption. In the present invention, a process has been developed for increasing the gas storage capacity and especially the storage capacity for CO2, certain MOFS, including a MOF called CuBTC, can be made to adsorb more CO2 than in its open-metal site form by simply titrating these open metal sites with water or other molecules before performing the CO2 adsorption measurement. In addition, other molecules can be used instead of or in addition to water to further enhance and/or modify the MOF adsorption performance for CO2 or other gases.